The present invention relates to a vent valve apparatus for use with a system for supplying breathable gas pressurised above atmospheric pressure to a human or animal.
The invention has been developed primarily for use in controlling the venting of washout gas in a continuous positive airway pressure (CPAP) gas delivery systems used, for example, in the treatment of obstructive sleep apnea (OSA) and similar sleep disordered breathing conditions. The invention may also be used in conjunction with suitable mask and gas delivery system for the application of assisted ventilation treatment.
The term xe2x80x9cmaskxe2x80x9d is herein intended to include face masks, nose masks, mouth masks, appendages in the vicinity of any of these masks and the like.
Treatment of OSA by CPAP flow generator systems involves the continuous delivery of air (or breathable gas) pressurised above atmospheric pressure to a patient""s airways via a conduit and a mask. CPAP pressures of 4 cm H2O to 22 cm H2O are typically used for treatment of sleep disordered breathing due to OSA and/or central apnea, depending on patient requirements.
Treatment pressures for assisted ventilation can range up to 32 cm H2O and beyond, depending on patient requirements.
For either the treatment of OSA or the application of assisted ventilation, the pressure of the gas delivered to patients can be constant level, bi-level (in synchronism with patient inspiration and expiration) or autosetting in level. Throughout this specification the reference to CPAP is intended to incorporate a reference to any one of, or combinations of, these forms of pressure delivery.
The prior art method for providing CPAP treatment includes a vent for gas washout of the gas flow. The vent is normally located at or near the mask or in the gas delivery conduit. The flow of gas through the vent is essential for removal of exhaled eases from the breathing circuit. Adequate gas washout is achieved by selecting a vent size and configuration that will allow a minimum safe gas flow at the lowest operating CPAP pressure, which, typically can be as low as, around 4 cm H2O for adults and 2 cm H2O in paediatric applications.
Existing vent configurations include single or multiple holes, foam diffusers, slots and combinations thereof. A reference herein to a vent may be understood to include a reference to one or more holes, foam diffusers, slots or any combination of them.
The flow of gas from the gas delivery system through the vent to atmosphere creates noise as the delivered gas, and upon expiration the patient expired gas including CO2, passes through the vent. Increasing CPAP pressure results in more gas passing through the vent which in turn creates more noise. Existing prior art vents produce excessive noise when CPAP pressures are raised above about 4 cm H2O. This noise can adversely affect patient and bed-partner comfort. Existing vents are also inefficient as they allow more gas through the vent than is required for adequate exhaust gas washout and thereby require the flow generator to provide more flow than is necessary in order to maintain the required treatment pressure. Further, where treatment as is being supplied, such as oxygen, surplus treatment gas is vented and thereby wasted unnecessarily, A similar waste occurs where the supplied gas is humidified.
It is an object of the present invention to overcome or at least ameliorate one or more of these deficiencies of the prior art.
Accordingly, the invention provides a vent valve apparatus for use with a system for supplying breathable gas pressurised above atmospheric pressure to a human or animal, the apparatus includes a gas washout vent, a vent valve adapted to progressively restrict the flow area of the washout vent, and a pressure sensitive vent valve control means adapted to progressively cause said vent valve to restrict the flow area of the gas washout vent in response to increases in the pressure of the gas supply, thereby substantially regulating the volumetric flow of gas and/or CO2 gas through the washout vent over a range of gas supply pressures.
Preferably, the system supplies breathable gas to a human patient.
The breathable gas is desirably air.
In an embodiment of the invention, the gas washout vent consists of an orifice having the shape of an outwardly diverging truncated cone and the vent valve is a complementary cone shaped plug. The rim of the cone shaped orifice acts as a valve seat for the cone shaped plug.
Desirably, the pressure sensitive vent valve control means includes an elastic diaphragm connected to the vent valve such that displacement of the diaphragm results in displacement of the vent valve. In an embodiment, the diaphragm is produced from a rubber or other elastic diaphragm stretched over a circular orifice. The vent valve is preferably connected to the rubber by a connection means that allows for the stroke of the vent valve to be adjusted.
The vent valve is desirably attached to the centre of the rubber diaphragm by a connection means such as a rod, the rod being operatively connected to both the vent valve and the diaphragm.
In a preferred embodiment, the vent valve and diaphragm are directly connected by the rod. The rod is preferably rigid, such that movement of the diaphragm is directly proportional to the movement of the vent valve.
In another embodiment, the connection means include a lever assembly connecting the vent valve to the diaphragm so as to permit differing ratios of movement and mechanical advantage of the vent valve relative to the diaphragm.
Such an assembly can include a tension means that opposes the movement of the diaphragm caused by increasing gas supply pressure so as to bias the vent valve to a position where the flow area of the orifice is maximised.
It will be understood that the orifice covered by the diaphragm need not be circular and can be any convenient shape.
In one preferred configuration, the vent valve apparatus is provided in a branch connection from a mask. In another preferred configuration, the vent valve apparatus is provided in an air supply conduit substantially adjacent the mask.
In a further embodiment, the vent valve apparatus includes an aerodynamic member such as an aerofoil or wing. The member is disposed in the air supply conduit by a biased pivot mechanism and has an axis of rotation transverse to the direction of gas flow. Gas passing through the conduit causes the aerodynamic member to generate lift and to be rotatably displaced in proportion to the velocity of the as flowing through the conduit, As the gas velocity increases the aerodynamic member produces increase lift and the resulting rotational displacement is transmitted to the vent valve which restricts the flow area of the washout vent. The vent valve is preferably biased open to a maximum washout vent flow area by a tension means such as a spring.
In a further preferred embodiment an aerodynamic member is suspended in a branch of the conduit and adapted for rotation about an axis transverse to the branch. Rotation around the axis is in response to the gas flow, which is proportional to the pressure in the conduit. The member is connected to the vent valve such that as the pressure in the conduit increases the rotational displacement of the aerodynamic member causes corresponding movement of the vent valve, resulting in a reduction of the flow area of the washout vent. A tension means is adapted to bias the vent valve, such as a spring, operates on the aerodynamic member to oppose the rotation caused by the gas flow on the aerodynamic member. The tension means biases the aerodynamic member in a rotational direction opposite to the direction induced by the gas flow entering the branch from the conduit.